FIG. 1 illustrates one configuration example of a certain ROADM node device in a mesh network.
In optical networks, an optical fiber is generally duplexed with a pair of optical fibers 1 so as to prevent the optical communication from being disconnected even when a break of an optical fiber or a failure of a transmission device occurs.
For example, in a method called 1+1 protection, optical signals are simultaneously transmitted in a direction of West→East through one of the pair of optical fibers 1 and in a direction of East→West through the other, and when the main optical fiber (West→East) is broken, the main optical fiber is instantaneously switched to the other optical signal (East→West), and thereby prevents a failure.
This node has a configuration having pairs optical fibers 1 in three directions (referred to as 3 Degree), in other words, the pairs of optical fibers 1 are laid toward three respective nodes positioned in the West, East, and North directions.
Hereinafter, an operation of this node will be described.
This node includes an Express Traffic device 2 and an Add/Drop Traffic device 3.
Herein, in the optical fiber of the pair of optical fibers 1 in which signals are transmitted in the direction of West→East, wavelength signals of approximately 100 waves are multiplexed, and are wave-split using a 1×N WSS (Wavelength Selective Switch). Two of output ports of the 1×N WSS are directed toward the East and the North, and are connected to 1×N WSSs that are disposed opposed to them. The remaining output ports are connected to two transponder banks (transponder bank # A4, transponder bank # B5) included in the Add/Drop Traffic device 3.
This allows an optical signal having a desired wavelength to be extracted, or an optical signal having a new wavelength to be inserted.
In this manner, in the Express Traffic device 2, the 1×N WSS devices are disposed opposed to each other, and ports thereof are connected to each other.
With the development of the optical networks in the future, the number of degrees tends to increase, and a larger number of 1×N WSSs become necessary.
Meanwhile, in the case of adding Degree, a 1×N WSS is newly installed, and optical patch fibers between the new and existing 1×N WSSs are required to be manually connected.
FIG. 2 illustrates an example of the configuration of an Add/Drop Traffic device.
In this example, a transponder bank is divided into a receiver (RX) bank 21 and a transmitter (TX) bank 22 for convenience, but actually is a transponder bank including a TX and RX bank.
Firstly, the receiver bank 21 will be described.
Optical signals (multiplexed wavelength signals) from the Express Traffic device 1 are amplified by optical amplifiers 23, and then are each divided by a 1×8 optical splitter 24 into eight branches, which are respectively inputted into 1×8 optical switches 25 connected to the respective receivers (RXs).
The 1×8 optical switch 25 uses a method of selecting optical signals of one degree from optical signals transmitted from the respective degrees, and further taking out a signal of one wavelength from the multiplexed optical signals by using a wavelength variable filter 26.
Meanwhile, in the transmitter (TX) bank 22, optical signals from the transmitters (TXs) are transmitted to a desired degree using the 1×8 optical switches 25.
In this manner, the degrees, the receivers (RXs), and the transmitters (TXs) are configured to be cross-connected by a method of multicast and select.
It should be noted that the receivers (RXs) including the wavelength variable filters 26 have a wavelength cross-connect configuration, and the transmitters (TXs) including no wavelength variable filters 26 are equivalent to an optical cross-connect configuration.
The method in this example is that the eight receivers (RXs) and the eight transmitters (TXs) are cross-connected, and this number is decided mainly depending on the amplification performance of the optical amplifiers.
Optical signals of about several tens to 100 waves are generally multiplexed per optical fiber.
Accordingly, in the case of 3 degree, by considering the duplexing, optical signals of about 600 waves at maximum in total are present.
Assuming that 50% of these optical signals are branched and inserted in the node, optical signals of about 300 wavelengths are required to be handled in the transponder bank.
In this example, since one transponder bank includes eight receivers (RXs) and eight transmitters (TXs), only eight waves can be handled per one transponder bank.
Thirty eight transponder banks are required for handling all optical signals of 300 wavelengths, which increases the size of the devices and the complication.
Moreover, the costly optical amplifiers 23, and many components including the 1×8 optical switches 25, the 1×8 optical splitters 24, and the optical filters 26 become necessary, so that there has been a problem of the upsizing and the increased cost of the devices as a whole.
To address this, an N×M wavelength cross-connect (WXC) device illustrated in FIG. 3 and an optical cross-connect (OXC) device illustrated in FIG. 5 have been proposed.
Firstly, the N×M wavelength cross-connect (WXC) device in FIG. 3 will be described (Non Patent Literature 1).
FIG. 3(a) is a top view, and FIG. 3(b) is a side view.
The N×M wavelength cross-connect (WXC) device includes an input port 34, an output port 35, lenses 33, gratings 32, switching engines 31, and a Fourier lens 36, which are arranged in one direction, and cross-connects optical signals for every wavelength using the two switching engines 31 (in this case, Liquid Crystal on Silicon (LCOS)).
Although this example is complicated, an optical system between the two switching engines 31 in a switching plane (FIG. 3(a)) is a basically Fourier optical system, and the number of ports M that can be handled is given by Equation (3).
                    [                  Equation          ⁢                                          ⁢          3                ]                                                            M        =                              Δθ            a                    ⁢                                                    f                ⁢                                                                  ⁢                π                            λ                                                          (        3        )            
Here, f indicates an equivalent focal length of the Fourier optical system, Δθ indicates a deflection angle of the light beam, a indicates a fill factor (a=R/w, where R is a half pitch of the light beam on the LCOS, and w is a spot radius of the light beam on the LCOS), and λ indicates a wavelength.
Here, a is about 1.5 to 2.0.
FIG. 4(a) illustrates a LCOS (Liquid Crystal On Silicon) that is a switching engine used in the related art.
In this LCOS, metal electrodes 42 are formed on an electronic circuit substrate 41, a transparent electrode 44 that is formed on a lower surface of an upper glass substrate 43, and liquid crystals 45 are inserted between the metal electrodes 42 and the transparent electrode 44. Then, when a voltage is applied to the metal electrodes 42, birefringent molecules of the liquid crystals 45 are rotated to provide a refractive index distribution in a stepped form pattern.
This controls the phase of incident light, thereby deflecting the light beam.
FIG. 4(c) illustrates a relation between the Blaze period N (the number of pixels for one period being 0 to 2π) of pixels (corresponding to the metal electrodes 42) of the LCOS and the loss.
When N becomes about 5 or less, the loss suddenly increases. Meanwhile, the deflection angle of the LCOS is expressed by Equation (4).
                    [                  Equation          ⁢                                          ⁢          4                ]                                                                      tan          ⁢                                          ⁢          θ                ≅                  λ                      N            ·            Δ                                              (        4        )            
Here, θ indicates the deflection angle, λ indicates the wavelength, N indicates the Blaze period, and Δ indicates the pixel pitch.
From FIG. 4(c), considering that N=about 5 or more is necessary for deflecting the light beam at the low loss, the pixel pitch Δ is necessary to be reduced in order to obtain a large deflection.
However, if the pixel pitch Δ, in other words, the pitch of the metal electrodes 42 is narrowed, the loss sharply increases due to the extended electric field (fringing effect) as illustrated in FIG. 4(b).
Therefore, the pixel pitch Δ is currently set to about 8 μm. The deflection angle θ is as small as about 2° from Equation (4), and it is impossible to make the deflection angle θ large.
FIG. 5(a) illustrates an example of a conventional optical cross-connect (OXC) device (Non Patent Literature 2).
In this example, two-dimensional MEMS mirrors are used to two-dimensionally control the deflection of light beams.
The optical cross-connect (OXC) device includes an optical fiber array 51, a micro lens array 52 for collimating, and two-dimensional MEMS mirrors 53.
Mirror angles of the two two-dimensional MEMS mirrors 53 are adjusted to optical cross-connect light beams between an input port and an output port.
The number of ports that can be handled is given by Equation (5).
                    [                  Equation          ⁢                                          ⁢          5                ]                                                            M        =                                            Δθ              2                                      9              ⁢                              α                2                                              ⁢                      (                                          f                ⁢                                                                  ⁢                π                            λ                        )                                              (        5        )            
Here, f indicates a focal length of the Fourier optical system, Δθ indicates a deflection angle of the light beam, a indicates a fill factor (a=R/w, wherein R is a radius of the MEMS mirror, and w is a spot radius of the light beam on the MEMS mirror), and λ indicates a wavelength.
Here, a is about 1.5 to 2.0.
FIG. 5(b) illustrates a cross section structure as an example of the MEMS mirror.
When the voltage is applied to drive electrodes 55 formed in a stepped shape, a mirror 54 inclines around a hinge 56.
If the mirror 54 inclines at a large angle, a Pull-in occurs in which the mirror 54 cannot return, and the inclination angle is limited to about ±1°. Thus, a deflection angle of this MEMS mirror is approximately ±2°, which is about twice of the inclination angle.
From the foregoing, the deflection angle by the LCOS or the MEMS mirror in the related art is several degrees at most, and the number of ports is estimated as about several tens at most in accordance with Equation (3) indicating the number of ports of the wavelength cross-connect device. Therefore, if the number of ports is increased, the devices are extremely large in size (increase in f).
Moreover, in the case of Equation (5) indicating the number of ports of the optical cross-connect device, the focal length of about 1 m is necessary in order to implement the port scale of 256×256 even by using the two-dimensional MEMS mirror, which results in the upsizing of the device and the increased size of the switching engine because the spot of light beam diffracts and becomes large, so that the cost is increased.
As a steering element that can have a large deflection angle, the steering element in which liquid crystal half-wavelength plates and polymer polarization gratings are alternately stacked is disclosed (Patent Literature 1).
FIG. 6 illustrates an example thereof.
This switching engine is obtained such that liquid crystal half-wavelength plates 61 and polymer polarization gratings 62 are alternately stacked.
The polarization grating 62 is formed such that the orientations of birefringent molecules arranged on a film in-plane (x-y plane) change in an x axis direction at a period ∧, as illustrated in FIG. 7.
A circular polarization state after the light passes through the polarization grating 62 is the reverse of a circular polarization state before the light is incident (the right-handed direction changes to the left-handed direction, the left-handed direction changes to the right-handed direction).
The light that is vertically (in a z axis direction) incident to this film deflects in the right and left direction (+X direction, −X direction) depending on the right-handed direction and left-handed direction of the circular polarization.
The deflection angle is given by the following Equation.
                    [                  Equation          ⁢                                          ⁢          6                ]                                                                      sin          ⁢                                          ⁢          θ                =                              m            ⁢                          λ              Λ                                +                      sin            ⁢                                                  ⁢                          θ              in                                                          (        6        )            
Here, λ indicates the wavelength, ∧ indicates the period of the polarization grating 62, θin indicates the incident angle, θ indicates the output polarization angle, and m indicates the order and has a value of ±1 in accordance with the polarization angle.
As for the polarization grating 62, there are two types of a liquid crystal type and a polymer type, the polymer type is a passive element, and the liquid crystal type can erase the polarization grating or reproduce the polarization grating, by changing the voltage.
The explanation is made referring back to FIG. 6.
Herein, the polarization gratings 62 of the passive type are used.
(a) illustrates an overall view, and (b) illustrates an operation view.
The method is that the direction (right-handed direction or left-handed direction) of the circular polarization of incident light is changed to switch the angle by 1×2 for each stage.
The polarization gratings 62 are stacked to allow many switching angles to be implemented, for example, when N polarization gratings 62 are stacked, 2N angles can be implemented.
Moreover, changing the periods of the polarization gratings 62 stacked can control the angle that allows the maximum deflection.
However, this conventional example indicates that the liquid crystal half-wavelength plate 61 can include one or many electrodes, however, this method is applied to the N×N wavelength cross-connect device or the N×N optical cross-connect device to cause the following problems, and it is difficult to introduce this method into practical systems.
(1) In a Case of the N×N Wavelength Cross-Connect Device
(i) The conventional example reports that one light beam is deflected, however, when this method is applied to the N×N wavelength cross-connect device, many light beams incident from many input ports (for example, 100 ports) and having different wavelengths (for example, 100 wavelengths) are required to be individually and independently switched. Moreover, regions to be switched of many light beams having different wavelengths are required to programmatically changed, because the number of wavelengths thereof and the wavelength widths thereof are temporally changed.
(ii) In the case of the N×N wavelength cross-connect device, light beams having various wavelengths are incident into this switching engine as described the above, and those are required to be switched for every wavelength. However, the periods ∧ are constant in the x axis direction in the conventional method, so that the polarization angle differs for every wavelength, thereby resulting in generation of the wavelength dependent loss.
(iii) This method is a method of conducting a 1×2 switch for each stage in tandem. The angle of switching changes by 2p (p is the number of plates stacked) but not in an analog fashion. The number of p is required to increase in order to switch the light beams in many ports by this method, however, the starting points of light beams in the final plane are spread out (Walk-off) due to the thicknesses of all the liquid crystal half-wavelength plates. This Walk-off causes the beams, which are originally required to be in parallel, to have different angles in the wavelength cross-connect device, thereby significantly increasing the loss.
(iv) It is difficult to implement the light beams having various magnitudes and the various numbers of light beams as one switching engine, and is impossible to implement a universal switching engine.
(2) In a Case where this Method is Applied to the N×N Optical Cross-Connect Device
(i) This method is a method of conducting a 1×2 switch for each stage in tandem. The angle of switching changes by 2N (N is the number of plates stacked) but not in an analog fashion. The number of N is required to increase in order to switch in many ports by this method, however, the starting points of light beams in the final plane are spread out (Walk-off) due to the thicknesses of the liquid crystal half-wavelength plates. This Walk-off causes the beams, which are originally in parallel, to have different angles in the optical cross-connect device, thereby significantly increasing the loss.
(ii) It is difficult to implement the light beams having various magnitudes and the various numbers of light beams as one switching engine, and is impossible to implement a universal switching engine.